Physical activity is well-recognized as a key risk factor for the management and prevention of mental ill-health, including anxiety and depression. The specific volumes, intensities or types of physical activity with the greatest impact on mental health are currently unclear. The current study sought to explore what aspects of physical activity may have positive or negative impacts on mental health and wellbeing. Focus group interviews were conducted with 10 adolescent females, and transcripts were analyzed using thematic analysis. Three higher-order themes, tenets of self-determination theory, were identified; autonomy, competence and relatedness. Single-factor sub-themes such as opportunity, journey to competence and facilitator of connection were identified as multi-factor sub-themes such as fun or enjoyment, and engagement in the activity. The perception of physical activity as an opportunity was identified as a key factor in contributing to positive mental health and wellbeing. It appears that autonomously motivated physical activity experiences provide the greatest levels of satisfaction for adolescents’ psychological needs and therefore, they are the most effective method of enhancing mental wellbeing through physical activity. Future physical activity experiences should include an element of choice along with opportunities to engage in social interaction alongside opportunities for progression and achievement, as these appear to provide the best environment to foster positive mental wellbeing in adolescents.